The proposed study is distinctive in studying known carriers of resistant Enterobacteriaceae. It examines both phenotypic and genotypic expansion in the AMR bacterial population as a function of antibiotic treatment, and examines the effect of antibiotics on the dissemination of resistance genes from species to species. It will examine CRE, important emerging pathogens, which are a significant threat to public health, but yet not well studied. The proposed study will use an interdisciplinary approach, combining PK/PD, phenotypic, and genetic data to understand the effect of antibiotics on the likelihood of spread of AMR. Most previous studies have focused on the effect of antibiotic agents on clinical detection, infection or acquisition of AMR among non-carriers67,68. Other studies have looked at the effects on colonisation-resistance mostly among healthy volunteers69. Very few studies have examined the effects of various antibiotic agents on carriers of ARB and those focused primarily on vancomycin resistant enterococci (VRE)70. No similar studies have been conducted with regard to ESBL or to CRE. Correlation with clinical factors and environmental contamination has not been determined in previously published studies. The proposed study also extends the analysis to the effect of antibiotic treatment on dissemination of resistance genes which may be of great importance as these events may lead to emergence of new AMR strains and species. The proposed study not only examines the effects of various antibiotic agents and classes, but also the effects of duration of treatment and PK/PD indices on amplification of ARB. The study will also examine the effect of antibiotics on the epidemicity and fitness of colonising strains